In recently developed systems for admixing parenteral solutions, liquids are extracted in precisely controlled volumes from inverted bottles, into which ambient air is admitted through air filters to replace liquids being pumped from the bottles and to prevent partial vacuums from forming above liquids in the bottles. The air filters are used to remove potential contaminants from ambient air being admitted into the bottles. Other components of such systems include cassettes having positive displacement pumps for pumping liquids in precisely controlled volumes.
To enable liquids to be thus extracted in precisely controlled quantities from the bottles, pneumatic flow resistances of certain components including the air filters and hydraulic flow resistances of other components including the cassettes must be precisely known. It is a practical necessity, therefore, to test all or statistically valid samples of those components.
One method known heretofore for testing pneumatic flow resistance of an air filter made of a hydrophobic material is to wet one side of such a filter, to apply air pressure to the other side, and to measure the pressure required to form a bubble on the wetted side. The method tends not to have repeatable results and is not useful for testing air filters made of hydrophilic materials.
Methods using liquids for testing hydraulic flow resistances of cassettes noted above have been known heretofore but are regarded as destructive. It is not practical to use such a method to sort such cassettes by hydraulic flow resistance.